VTA ST-120 Speaker problem - solved !

Hello All,
I built a VTA ST-120 from kit more than a year ago, have been super happy with it. The STA-120 seems to
be causing some damage to the bass speaker driver, the  starts to  distorting the bass a lot, I replaced
my Bose (book-shelf) after about 3-months with a used JBL pair, that seems to work for about 8-9 months
now I hear the bass distortion again . Any suggestions on how to approach the problem ...

It is wonderful amplifier , play it a lot.

Best Regards
Sandeep

No comment on the power-handling capacity (negligible) of Bose (all) and JBL (most) speakers.

However, this does seem to indicate a classic case of sub-sonic oscillation coming from somewhere within your system. If you play a lot of vinyl, I would start there - see "test" later. But, first try this:

With no signal, but with the Volume-control on your system about where you typically run, measure AC volts at the speaker terminals (speakers connected, of course). If you are getting _ANY_ voltage, that is indicative of oscillation. If your VOM can measure frequency, try to determine that as well.

Test: IF you play a lot of vinyl:

a) VC at 0.
b) Record on TT, but TT not turning.
c) Place stylus on record - again _NOT_ turning.
d) Turn up volume slowly to your typical listening level - KEEP YOUR HAD ON IT!!

If you start to get feedback through the TT, turn it back down right away. And then, work on either a better location for your TT, or some way to isolate it from the environment so that those oscillations go away. If you do not get feedback, at least be assured that your TT and stylus are good, and such oscillations would be coming from somewhere else. It won't be clipping artifacts, as tube amps clip very softly, and do not pass DC.

Best of luck with it!

Hello All,

Thank you for analysis suggestions ,

Peter W - I play vinyl almost exclusively , I tried your experiment, there is some feed back  , although at much  higher volumes  than my normal listening level. I have ordered some vibration isolation foots for my Turntable.

Bob, the JBL speakers I am using are indeed very old L56 I will not be surprised if they are 50+ years old.


The woofer driver is damaged, I plan to replace the innards of the speaker (keeping the JBL enclosure) , so I am looking for recommendations for a  2-WAY crossover and 12" woofer that can take the power output of my beautiful STA-120
Help in the selection will be much appreciated.

Best Regards
Sandeep

Sandeep, you may want to contact Simply Speakers. These are the folks I have used for my EV needs in my Khorns. Clearwater, Florida. Search for their website. Dale

A few random thoughts:

1) Have you tested your speakers using another amplifier?
2) Have you inspected the woofer suspension (surrounds) for breaks/tears/cracks?
3) Have you tested your setup using a source other than a record player? It would be good to bypass your phono and line stages to isolate any potential instabilities there.
4) Both channels do this? I believe your amplifier has left/right pentode/triode switches. Does switch position have any effect on the distortion?

...J

Some "Stuff". Assuming a properly operating system with properly operating components, of course:

Generally, it is too little power rather than too much power that damages systems. With (some) solid-state systems, that is due to amps delivering DC to the speaker when clipping hard. With (some) tube systems run at top volume, that is because simply too much power is going into the voice-coil at some specific frequency. Even 25 watts of power delivered to a good speaker via a 1000 HZ sine wave will melt the mid-range voice-coil in a very few seconds.

60 watts is not a lot of power. Certainly not enough to cause damage under normal conditions. I routinely drive my pair of AR4X speakers with a 120 WPS/RMS amp at substantial volumes, and have done so for years without any damage at all. And I have driven an AR Athena sub-sat from my Scott LK150 at 75 WPC.

And as much as I may disparage JBL or Bose - I really do not think the speakers are simply being over-driven, as much as I think there is something else going on. And why I am asking for direct voltage measurements - that should tell the tale. And, if a frequency counter might get involved - even better. Good diagnoses require good data.

Not mentioned is equalization. Depending on your tube selection, the ST120 can deliver a LOT of clean power. Too much boost on the bottom can and will distort (and even destroy) speakers if not addressed properly.

If you've got a laptop handy, get a copy of Room EQ Wizard (free) and a calibrated mike and see if you got some serious spikes in the results. I wouldn't be surprised to find you're over-boosting a lot of the bottom looking for a specific instrument in a narrow frequency band.

And yes, the smaller the speaker, the greater the risks of distortion. If you're limited to bookshelf size, I'd suggest a pass through type sub to handle the low stuff. Doesn't need to be powered as the ST120 can provide more than enough with plenty to spare driving a passive system.

Do the initial tests with the mike up close to the drivers. This should tell you what the speakers have to work with. Follow up with the mike in the primary listening position to see what the room itself is doing to the sound. There too, you can be over-compensating for a bottom end drop off there, where it's most important to you.

As mentioned too little power generally will cause more damage than too much due to clipping.  If these speakers are very old as Bob asked, they may require refoam, recone, or voice coil repairs.  Have you done a visual inspection of the bass drivers removed from the cabinet?  It might be worth the check to rule out damage to the drivers.  Also do what Peter suggested, direct voltage measurements to rule out systemic oscillation.  One or both of these checks should tell you what is happening

Thank you all gentlemen. The problem is resolved
The culprit was my equalizer , it was boosting the bass < 30Hz way more, causing it to clip.
Measuring through Room Equalizer immediately pointed out the problem .
Once again thank you all for your excellent analysis suggestions.
Learnt a lot.

Best Regards
Sandeep